The use of subsurface well bore devices such as plugs, safety valves, packers, and the like is well-known in the oil field art. Such tools are generally lowered downhole by either a wireline or a working string and are typically configured with a fishing neck 318 to facilitate recovery at a later date. The tool is set at a chosen location and released, allowing the wireline or work string to be retrieved.
Conventional methods of setting and retrieving such tools are performed mechanically by a work string or wireline or by electrically actuated power units. Electrically actuated power units commonly utilize a conductor in the wireline to accomplish actuation by surface power, after the tool is properly positioned. Self-contained downhole power units, referred to as "DPUs," do not require electrical power from the surface and therefore, permit using a slickline rather than a wireline. The use of downhole power units and slicklines is desirable because of their relative speed and efficiency of use and because slickline equipment is more widely available than wireline equipment.
When a downhole power unit has been placed in the desired downhole working position, it may be actuated in several different ways. A timer, accelerometer, pressure sensor, or combination of such devices can be used to initiate actuation. A pressure actuated downhole power unit may be activated by a pressure sensor, preset to sense a given well bore pressure, corresponding to the depth of the planned downhole operation. Once the expected pressure is sensed, a timer delays actuation of the downhole power unit sufficiently to allow for its final positioning. Although this method is successful in most installations, sometimes downhole conditions defy prediction and a miscalculation causes the operation to fail.
Therefore, there is need of a method and apparatus to provide for more positive and timely actuation of a downhole power unit than is possible by currently practiced methods and available apparatus.